Electric dynamometers are often used to facilitate motor vehicle testing and to enable laboratory measurement of various performance criteria such as fuel economy and engine exhaust emissions. In some applications, the dynamometer --an engine dynamometer --directly engages the output shaft of an engine; in others, the dynamometer --a chassis dynamometer --engages the tires of an entire vehicle. In either case, the dynamometer is controlled to develop torque which opposes the torque developed by the vehicle engine, and the engine behaves as though it were propelling a vehicle down the road. Fuel consumption and exhaust emission measurements obtained under such conditions closely correspond to those which would be obtained under actual driving conditions.
In one method of dynamometer control, off-line control, the engine speed and torque over time required to drive the vehicle along a predetermined course such as the Federal Urban Driving Schedule are calculated or otherwise determined, and then imposed on the engine and dynamometer in the laboratory test cell. The predetermined torque vs. time characteristic is imposed on the engine with a closed loop engine throttle controller, and the predetermined speed vs. time characteristic is imposed on the engine with a closed loop dynamometer speed controller. This method is not entirely satisfactory because it assumes, simplistically, a causal and predetermined relationship between throttle position and engine speed.
In another method of dynamometer control, on-line control, the engine output torque is measured and used in conjunction with a model of the vehicle inertias to compute the engine speed which would occur were the engine propelling a vehicle down the road. The computed speed is imposed on the engine on a real time basis with a closed loop dynamometer speed control. To permit economy or emissions measurement for a predetermined driving schedule, the engine throttle is controlled to achieve the scheduled vehicle speed. This method assures causality between throttle position and engine speed, and is therefore considered superior to the off-line method.
Prior work with the on-line method of control has been concerned primarily with chassis or roll dynamometers, where the dynamometer engages the tires of the vehicle and is controlled to emulate the vehicle inertia and road load. Heretofore, the on-line method of control has not been used in connection with an engine dynamometer to emulate the entire vehicle drivetrain in addition to vehicle inertia and road load.
Accordingly, it is an object of this invention to provide an improved on-line emulation system for controlling the operation of an engine dynamometer as a function of measured engine torque, wherein the emulation includes the operation of conventional drivetrain elements including a fluid torque converter and clutch, and an automatic shift multiple speed ratio transmission. Since no transmission is installed between the engine and the dynamometer, the dynamometer need only contend with engine output torque as opposed to the significantly higher transmission output torque.
Nonlinear differential equations relating the ability of the drivetrain input to accelerate the engine inertia and the ability of the drivetrain output to overcome the road load and accelerate the vehicle inertia are defined and updated in relation to measured engine output torque. Drivetrain elements including a fluidic torque converter and shiftable ratio transmission are modeled and used to couple the engine and vehicle differential equations. The drivetrain input torque is determined as a function of the measured engine output torque and the load that would be imposed on the engine by the torque converter and the drivetrain output torque is determined as a function of the transmission output torque and the road load of the vehicle. The coupled differential equations are periodically integrated forward in time assuming substantially constant engine output torque during the integration period using a fourth-order RUNGE-KUTTA technique. The integration provides future desired values of engine and vehicle speed --values which would occur were the engine driving the simulated vehicle through the simulated drivetrain components. The desired (predicted) engine speed becomes a speed command for the dynamometer, and the desired (predicted) vehicle speed can be used as a simulated vehicle speed if it is desired to make the vehicle speed conform to a predefined schedule.
The emulation system of this invention permits realistic testing of an engine on any drive schedule, and is therefore well suited to engine development work. In addition, the effect of adjustments in transmission calibration, including shift pattern, can conveniently be determined by appropriately adjusting the emulation parameters.